Reinforced plastic cylindrical storage tanks are generally initially made with relatively thin, flexible walls to reduce the amount of resin required, and subsequently provided with external annular reinforcing ribs to increase rigidity. The ribs are formed of reinforced resin over cardboard rib forms. Prior to our invention, rib forms such as shown in U.S. Pat. No. 3,700,512, issued to Lee E. Pearson and Jeri O. Clark on Oct. 24, 1972, were most generally used. The prior rib form was three-sided in cross section, i.e., trapezoidal with the longer of the two parallel sides omitted, and was stapled to trapezoidal wood or plastic blocks at intervals for the retaining of its shape.